THE TRACKING SOLAR COOKER
Copyright 1987, 1988, 1990, 1992, 1993 by C. Alan Nichols P.E., C.E.M. Tucson, Arizona
All rights reserved
The concept of solar cooking began over 220 years ago and was used by the French Foreign Legion starting in the 1870's. All solar cookers work on the principle of concentrating the direct solar rays to raise food or water to cookingtemperatures. Cooking temperatures begin at about 150 degrees F. although temperatures of 250 to 400 degrees F. are preferred. Open reflector type solar cookers focus the sun's rays on open cooking pots or pans. Solar ovens trap the sun's heat inside insulated boxes with transparent lids. Most solar ovens are variations of the bread box type developed by two Arizona women, Barbara Kerr and Sherry Cole.These simple box cookers created in the 1970's are now being used world wide to overcome food shortages in developing countries. In places like Eastern Africa native women travel 20 to 30 miles to gather a two day supply of cooking fuel. In other parts of the world increasingly high rates of malnutrition are caused by a lack of fuel. Basic grain foods cannot be cooked without cooking fuel and waterinfected with chronic bacteria must be heated before drinking. Continued gathering of wood cooking fuel by chopping down trees has resulted in eroded hill sides with loss of precious topsoil thus reducing their ability to grow food. In America we concern ourselves with such things as operating costs, air pollution, acid rain and the green house effect. Let's say it takes one hour to cook a pot ofbeans on an electric stove using one kilowatt. The coal fired power plant that supplied the electricity consumed one pound of coal and released 17.5 cubic feet or two pounds of CO2. The power plant also consumed 0.7 gallons of ground water and released traces of SO2 as acid. Suppose you are cooking outside. Five pounds of steaks on a grill will use a ten pound bag of charcoal and five ounces oflighter fluid. This fire will produce approximately 160 cubic feet or fifteen pounds of CO2 and untold air pollution. Wood fires are even worse. Cooking in the home averages over 100 hours a year consuming approximately 1,175 kilowatt hours. At a cost of $0.10 a kilowatt hour this amounts to $117 a year. During the summer cooking adds $50 to the air conditioning bill bringing the total to $167 ayear. In Arizona, solar cooking can replace 70% of the cost of cooking. This will save 1,675 pounds of coal and 3,000 pounds of CO2 generation from coal fired electric utilities. Clearly solar cooking has come of age.
HOW SOLAR OVENS WORK A solar oven's cooking temperature is reached when the solar gain equals the heat losses. Thus an oven's cooking temperature is a balance between solargain and heat losses. Heat losses fall into five categories: 1. Reflective losses 2. Absorption Losses 3. Transmitted Losses 4. Leakage Losses 5. Food losses (Heat Lost To Cooking) Solar gain is a factor of the total area exposed to the sun and the effectiveness of collection. For the reflectors this property is acceptable but for the cover glass, light must enter the cooking chamber, or the mirrorsare of no use. As a stone skipping across water, sunlight will skip off the surface of glass without penetrating if the angle is too shallow. An angle of about 30 degrees from the vertical for the mirrors or reflectors yield good penetration with a healthy spread for an optimum use of materials. As it turns out a reflector equal to the width of the cooking chamber produces an optimum design foruse of materials. With such a fortunate set of circumstance the reflectors can be folded to cover the cooking chamber making the cooker portable!
Mirrors have a reflection of about 99% but lose 5% each time the light passes through the glass. Polished metal surfaces have about 90% reflection but no losses due to glass. The result is that each behaves about the same. Light striking the far edge...
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